The invention refers to a process for the preparation of thiuram disulfides substituted with aliphatic, cycloaliphatic, araliphatic and/or aromatic hydrocarbon radicals by reacting a suitably substituted secondary amine with carbon disulfide in the presence of oxygen or a gas containing oxygen, a metalliferous catalyst, and a tertiary amine or ammonia.
Making use of known processes, thiuram disulfides can be obtained by the oxidative dimerization of salts of substituted dithiocarbamic acids. Hydrogen peroxide, nitrogen dioxide, chlorine, bromine, iodine, ozone, oxygen, sodium nitrite, sodium hypochlorite, sulfur chlorides, potassium perbromate, selenic acid, or ammonium persulfate are used as oxidant. Tetramethyl thiuram disulfide, one of the most important representatives of this category of compounds, is made on an industrial scale by means of a two-stage process. In the first stage, dimethylamine and carbon disulfide in aqueous sodium hydroxide are reacted to form sodium-N,N-dimethyl dithiocarbonate. In the second stage the dithiocarbamate is oxidized with hydrogen peroxide in the presence of sulfuric acid (Bios 1150, Fiat 1018), with chlorine (U.S. Pat. Nos. 2,751,514 and 2,751,416), or electrolytically (German patent application disclosure Nos. 28 02 260 and 28 03 591).
In the process of German Pat. No. 12 26 564, a secondary alkyl-, aryl- or alkylarylamine is reacted with carbon disulfide in an aqueous or non-aqueous medium and in the presence of an oxygen-containing gas and a metal catalyst to form substituted thiuram disulfide. A sulfonated or carboxylated metal phthalocyanine of the 8th group of the periodic system, as for example cobalt phthalocyanine, is used as catalyst. In this process, the yield is relatively low; at best, it is about 25% of theoretical. When aromatic amines such as diphenylamine are used, the process of German Pat. No. 12 26 564 does not result in the formation of thiuram disulfide. In addition, the preparation and industrial use of the cobalt catalyst are problematical.
The use of a metalliferous catalyst in the oxidation of alkali salts of substituted dithiocarbamic acids with oxygen is also known. According to the process of German published application No. 11 65 011, the oxidation is carried out in an aqueous solution of a sulfonated or carboxylated Group VIII metal phthalocyanine at a pH of about 7 to 12. However, materials used in this process add to its expense and form unusable byproducts. Lye is needed for the preparation of the dithiocarbamates and hydrochloric acid is required for pH adjustment, and these form unusable sodium chloride. Further, the industrial preparation and application of these Group VIII metalliferous catalysts is problematical.
The use of an ammonium salt of dithiocarbamic acid, instead of the alkali salts, is also already known. In the process of German patent application disclosure No. 25 27 898, ammonium dimethyldithiocarbamate is oxidized by means of hydrogen peroxide in an aqueous solution of sulfuric acid at a pH of from 5 to 7 to yield a suspension of solid tetramethyl thiuram disulfide in an aqueous ammonium sulfate solution. After the solid tetramethyl thiuram disulfide has been filtered off the resulting filtrate must be concentrated down to the solubility limit of the ammonium sulfate, resulting in its precipitation. The ammonium sulfate could be used as a fertilizer, but only if the adhering dithiocarbamate is removed. This makes the ammonium sulfate an undesirable byproduct.
Thus, there is a need for a process for the preparation of a thiuram disulfide substituted with aliphatic, cycloaliphatic, araliphatic and/or aromatic hydrocarbon radicals, by reacting a suitably substituted secondary amine with carbon disulfide in a solvent and in the presence of oxygen or a gas containing oxygen and a metalliferous catalyst. The reaction is carried out at temperatures of from 0.degree. to 200.degree. C., and a preferred embodiment comprises reacting the carbon disulfide and the secondary amine in a molar ratio of 1.0 to 1.2:1 in the presence of a tertiary amine or ammonia, oxygen or a gas containing oxygen, and the metalliferous catalyst. Another embodiment comprises reacting equimolar quantities of carbon disulfide, the secondary amine, and a tertiary amine or ammonia, and thereafter reacting the resulting reaction mixture with carbon disulfide in the presence of the metalliferous catalyst and oxygen or a gas containing oxygen. A still further embodiment comprises reacting carbon disulfide, the secondary amine, and a tertiary amine or ammonia to form the dithiocarbamate, which is subsequently isolated and reacted in the presence of the metalliferous catalyst and oxygen or a gas containing oxygen. The catalyst may be selected from one or more of the group including copper, silver, gold, zinc, cadmium, mercury, lanthanum, cerium, titanium, zirconium, vanadium, nicobium, tantalum, chromium, molybdenum, tungsten, uranium, manganese, rhenium, iron, cobalt, as well as nickel, or derivatives of the mentioned metals.
Suitable aliphatically substituted secondary amines include: dimethylamine, diethylamine, dipropylamine, diisopropylamine, dibutylamine, di-sec-butylamine, di-tert.-butylamine, di-(2-methylpropyl)-amine, dipentylamine, di-(1-methylbutyl)-amine, di-(2-methylbutyl)-amine, di-(3-methylbutyl)-amine, di-(1,1-methylpropyl)-amine, di-(2,2-dimethylpropyl)-amine, di-(1,2-dimethylpropyl)-amine, dihexyl-amine, di-(1-methylpentyl)-amine, di-(2-methylpentyl)-amine, di-(3-methylpentyl)-amine, di-(3-ethylpentyl)-amine, di-(1,1-dimethylbutyl)-amine, di-(2,2-dimethylbutyl)-amine, di-(3,3-dimethylbutyl)-amine, di-(2,3-dimethylbutyl)-amine, di-(1-ethylbutyl)-amine, di-(2-ethylbutyl)-amine, diheptylamine, di-(1-methylhexyl)-amine, di-(2-methylhexyl)-amine, di-(3-methylhexyl)-amine, di-(4-methylhexyl)-amine, di-(5-methylhexyl)-amine, di-(1-ethylpentyl) amine, di-(2-ethylpentyl)-amine, di-(3-ethylpentyl)-amine, dioctylamine, di-(1-methylheptyl)-amine, di-(2-methylheptyl)-amine, di-(3-methylheptyl)-amine, di-(4-methylheptyl)-amine, di-(5-methylheptyl)-amine, di-(6-hexyl)-amine, di-(3-ethylhexyl)-amine, di-(4-ethylhexyl)-amine, methylethylamine, ethylbutylamine, dilaurylamine, didodecylamine, ditridecylamine, dipalmitylamine, distearylamine and dioleylamine.
Suitable aromatically-substituted secondary amines include: diphenylamine, 4,4'-dimethyldiphenylamine, 3,3'-dimethyldiphenyl-amine, 2,2'-dimethyldiphenylamine, as well as alkylarylamines, such as N-methylaniline, N-ethylaniline, N-propylaniline, N-isopropylaniline, N-butylaniline, N-sec.-butylaniline, N-tert.-butylaniline, N-pentylaniline, N-(1-methylbutyl)-aniline, N-(2-methylbutyl)-aniline, N-(3-methylbutyl)-aniline, N-(1,1-dimethylpropyl)-aniline, N-(2,2-dimethylpropyl)-aniline, N-(1,2-dimethylpropyl)-aniline, N-(1-methylpropyl)-aniline, N-(2-methylpentyl)-aniline, N-(3-methylpentyl)-aniline, N-(4-methylpentyl)-aniline, N-(1,1-dimethylbutyl)-aniline, N-(2,2-dimethylbutyl)-aniline, N-(3,3-dimethylbutyl)-aniline, N-(2,3-dimethylbutyl)-aniline, N-(1-ethylbutyl)-aniline, N-(2-ethylbutyl)-aniline, N-heptylaniline, N-(1-methylhexyl)-aniline, N-(2-methylhexyl)-aniline, N-(3-methylhexyl)-aniline, N-(1-ethylpentyl)-aniline, N-(2-ethylpentyl)-aniline, N-(3-ethylpentyl)-aniline, N-octyl-aniline, N-(1-methylpentyl)-aniline, N-(2-methylheptyl)-aniline, N-(4-methylheptyl)-aniline, N-(4-methylpentyl)-aniline, N-(5-methylheptyl)-aniline, N-(6-methylheptyl)-aniline, N-(1-ethylhexyl)-aniline, N-(2-ethylhexyl)-aniline, N-(3-ethylhexyl)-aniline, N-(3-ethylhexyl)-aniline, as well as the corresponding alkyl-naphthylamines.
Suitable araliphatic secondary amines are the listed aliphatic and cycloaliphatic amines, in which one or several hydrogen atoms located on the hydrocarbon radicals are substituted by aryl radicals, as for example the following: dibenzylamine, di-(phenylethyl)-amine, di(2-phenylpropyl)-amine, di-(3-phenylpropyl)-amine, N-methylbenzylamine, N-ethylbenzyl-amine, N-propylbenzylzmine.
Examples of suitable cycloaliphatically-substituted secondary amines are the compounds hydrated in the nucleus corresponding to the above-mentioned aromatically-substituted secondary amines, as well as corresponding amines with cyclobutyl, cyclopentyl, cycloheptyl, or cyclooctyl substituents. As already stated above, the substituents of the secondary amine may be identical or different. They may, however, also be concyclic amines such as morpholine, piperidine, pyrrolidine and their derivatives.
Suitable tertiary amines are selected from the aliphatic, cycloaliphatic, aromatic and heterocylic amines, such as trimethylamine, triethylamine, tri-n-propylamine, tri-n-butylamine, n-octyl-dimethylamine, di-isopropyl-ethylamine, propyldimethylamine, ethyl-dimethylamine, isopropyl-dimethylamine, butyl-dimethylamine, N-methylpyrrolidine, N-dimethylaminopyridine and 1,4-diazobicyclo-(2,2,2)-octane.
In the process pursuant to the invention, the oxidant used is oxygen or a gas containing oxygen, such as air. Non-aqueous solvents suitable for use in the process pursuant to the invention include aromatic hydrocarbons such as benzene, toluene, xylene, nitrobenzene; aliphatic esters; alkylether; lower alcohols, such as methanol, ethanol, isopropanol, n-propanol, n-butanol, t-butanol and amyl alcohol; chlorinated hydrocarbons, such as dichloromethane, chloroform, dichloroethane, trichloroethane; and aprotic solvents, such as dimethyl formamide, acetonitrile, dimethylacetamide, dimethyl sulfoxide and hexamethyl phosphoric triamide. Suitable aqueous solvents include water/alcohol mixtures. High yields and selectivities may be obtained in pure water, but in general the reaction rate in water is slower than in the above-mentioned non-aqueous solvents. Preferred solvents include aromatic hydrocarbons, low alcohols and alcohol/water mixtures.
The process pursuant to the invention is carried out at temperatures in the range from 0.degree. to 200.degree. C., preferably from 20.degree. to 90.degree. C. Temperatures below 90.degree. C. are preferred for reasons of economic and safety. Preferred oxygen pressures or partial oxygen pressures are those no less than 0.1 bar. As is to be expected, the reaction rate increases with rising oxygen pressures.
The metals listed in the patent claims, or their derivatives, are used as metalliferous catalysts. In addition to the claimed catalysts, all other metals of the sub-groups of the periodic system of elements and their derivatives are suitable, but are not preferred for reasons of cost. Excellent metalliferous catalysts include cerium, manganese, copper, iron, cobalt, molybdenum or vanadium in elemental form, as salts, oxides, or complexes, or in the form of their organic compounds. Among the preferred metals or their derivatives, copper, manganese and cerium are more catalytically effective than iron, cobalt, molybdenum and vanadium.
Elementary copper is preferably used in the form of copper powder. Copper compounds to be considered are all mono- or divalent inorganic, organic, simple, or complex copper salts. Examples of suitable monovalent copper salts are copper(I) chloride, bromide and iodide, addition compounds of these copper(I) halides with carbon monoxide, complex copper(I) salts, such as the alkali chlorocuprates, complex ammoniacates of copper(I) cyanide, such as cyanocuprates, e.g. potassium tricyanocuprate(I), double salts with copper(I) thiocyanate, copper(I) acetate, copper(I) sulfide and complex double sulfides of copper(I) sulfide and alkali polysulfides. Examples of suitable copper(II) salts are copper(II) chloride, bromide, sulfide, nitrite, thiocyanate, or cyanide, Cu(II) salts of carboxylic acids, such as copper(II) acetate, copper dithiocarbamate, as well as the complex ammoniacates of copper(II) salts. Copper(I) oxide is also very well suited as catalyst.
Suitable manganese-containing catalysts include powdered manganese, manganese dioxide, potassium permanganates, manganese acetate, manganese dithiocarbamates, and the manganese derivatives corresponding to the above-mentioned copper compounds. Suitable cerium catalysts include metallic cerium, cerium dioxide, cerium(III) chloride, cerium chloride, cerium chlorocomplex salts, cerium nitrate, cerium nitrato salts, cerium sulfate, cerium carbonate, cerium oxalate and the cerium sulfides.
Examples of iron catalysts are the known iron oxides, iron(II) and iron(III) salts, and complex iron salts. Suitable vanadium catalysts include the vanadium oxides, chlorides and sulfates, and the known double and complex salts. Suitable cobalt catalysts are the known cobalt oxides, cobalt (II) salts, and the complex salts.
Finally, the suitable molybdenum catalysts include molybdenium oxides, chlorides, sulfides and fluorides, the molybdates, and the known complex acido salts. Mixtures of several of the above-mentioned catalysts may also be used.
The required quantity of metalliferous catalyst is surprisingly small. Preferably, it is within a range from 0.01 to 5 millimoles per mole of secondary amine. Smaller catalyst quantities can also be used, but the reaction rate is thereby slowed. Larger quantities of catalysts should be avoided, because there is a danger that such larger quantities could precipitate and contaminate the reaction product.
In principle, the process according to the present invention may be carried out according to three procedures. In the first procedure, equimolar quantities of the secondary amine and carbon disulfide are reacted with oxygen in the presence of the metalliferous catalyst and the tertiary amine or ammonia to form thiuram disulfide. The quantity of tertiary amine can be varied within wide limits, from catalytic to stoichiometric quantities. The tertiary amine may also function as a solvent, in which case it may be used in quantities corresponding to or even exceeding the quantity of solvent normally used. In the second procedure, equimolar quantities of carbon disulfide, secondary amine, and the tertiary amine or ammonia are reacted to form a mixture of intermediate products consisting essentially of the corresponding quaternary ammonium salt or ammonium salt of dithiocarbamic acid. This mixture of reaction products is then reacted with oxygen in the presence of the metalliferous catalyst, and it is thereby not necessary to separate the intermediate product(s) before they are reacted further. In the third procedure, equimolar quantities of the secondary amine and the tertiary amine or ammonia are dissolved in a suitable solvent, such as water or an alcohol. An equimolar amount of carbon disulfide, if necessary also dissolved, is then added. The reaction proceeds quickly and in most cases ends after a few minutes, thereby forming the quaternary ammonium dithiocarbamate or the ammonium dithiocarbamate. The solvent quantity is selected in an amount sufficient to ensure that the salt will precipitate as completely as possible so as to facilitate its filtration. In the final reaction step, the dithiocarbamate is reacted with oxygen in the presence of the metalliferous catalyst to form the thiuram disulfide. In the case of the first two procedures, the carbon disulfide is used in at least stoichiometric quantities. Using a 1 to 20 mole percent excess of carbon disulfide increases the thiuram disulfide yield and the reaction selectivity.
The reaction time depends upon the processing conditions and may range from a few minutes to three hours under the preferred temperature and oxygen pressure conditions. The present process comprises forcing oxygen or the gas containing oxygen onto the reaction solution at the given pressure and temperature conditions, or by conducting it into or through the reaction solution. Depending upon the procedure selected, the reaction mixture will consist of solvent, carbon disulfide, secondary amine, tertiary amine or ammonia, and metalliferous catalyst; or of solvent, metalliferous catalyst and quaternary ammonium dithiocarbamate; or of the reaction mixture obtained by reacting secondary amine, tertiary amine or ammonia, and carbon disulfide in a solvent, and a metalliferous catalyst. In most cases, as with tetramethyl thiuram disulfide, the end product precipitates immediately from the reaction mixture and can be filtered off. In other cases, the desired end product is obtained when the reaction mixture is cooled or concentrated. Liquid products are obtained in pure form by distillation or extraction.
In an industrial-scale process pursuant to the invention it is advantageous to circulate the mother liquor so as to obviate the need for constant catalyst replenishment. For example, it is possible to run more than ten high yielding reaction cycles without any loss of catalyst activity. Practically quantitative yields and selectivities of more than 99% can be obtained with the present process. The products obtained have a high degree of purity and can, as a rule, be used as intended without purification.
Compared to the known two-stage process in which the dithiocarbamate is synthesized first, the single-stage process is economical and environmentally less dangerous, since no auxiliary materials are consumed and no by-products are formed. Compared with the single-stage process of German Pat. No. 12 26 564, the present process uses very simple and inexpensive catalysts. Further, soluble catalysts are used in the industrial execution of the process pursuant to the invention, and the catalysts can be circulated several times with the mother liquor without loss of activity, resulting in practically quantitative yields. The thiuram disulfides made pursuant to the invention are used as vulcanization accelerators for synthetic and natural rubber and as agricultural chemicals.